It has become an important work in recent years to make positive actions against protection of the earth environment. Since energy saving is an urgent demand for electric home appliances, it is an important task to improve efficiency of heat insulation for the products such as refrigerators and hot/cold preserving boxes. In addition, high performance of insulating material also becomes inevitable for a variety of other home appliances and electronic apparatuses in order to facilitate energy saving.
Furthermore, as a recently occurring problem in notebook computers, heat generated within a computer is transferred to a surface of the device enclosure, and the heat of the enclosure surface in contact with a user's body for a long time gives an uncomfortable feeling to the user when the surface temperature rises. Insulating material having good insulating property is also desired in this instance.
Vacuum heat insulator is available as means to address the problems of the above kind. Japanese Patent Laid-Open Publications, Nos. S57-173689 and S61-144492, for example, disclose vacuum heat insulators that use inorganic powder. They describe methods of obtaining vacuum heat insulator by filling a plastic film envelope with inorganic powder having a particle diameter of 1 μm or smaller, and hermetically sealing it after an evacuation of its interior.
However, the above prior art technique has problems such as impairment of work environment due to powdery dust and complication of the manufacturing process, because of its use of fine powder.
In addition, Japanese Patent Laid-Open Publication, No. 2000-97390 discloses a method of providing vacuum heat insulator with flexibility to reduce leakage of heat from a sealing area of the vacuum heat insulator.
However, in order to provide flexibility to the vacuum heat insulator with the prior art technique, it is necessary that a core retain its shape after it is compression-formed. The prior art technique has not been employed, however, if the core does not maintain its own shape. In particular, when a sheet-formed material consisting of inorganic fibers is used as the core, the prior art technique has not been effective to provide the required flexibility since the inorganic fibers are broken by the pressure and the core disintegrates in the compression forming process.
Numerous attempts have been done to address these problems, and vacuum heat insulators using fibrous material in particular have been proposed.
According to Japanese Patent Laid-Open Publication, No. S30-3139, for instance, a kind of vacuum heat insulator is proposed, in that glass fibers having fiber diameter of 250 μm or smaller are used as a core, and its interior kept in vacuum of 0.75 Pa or less. Also, Japanese Patent Laid-Open Publication, No. S60-208226 describes a technique of using fibers as a core of the vacuum heat insulator, in which thin inorganic fibers are laminated randomly in a direction orthogonal to a direction of heat transfer, and binding fibers are sewn in up to halfway.
Furthermore, Japanese Patent Laid-Open Publication, No. H9-138058 discloses a method of fixing fibers with a binder, as another known example. This method is to form fibrous material such as glass wool using an organic binder, for use as a core of the vacuum heat insulator.
However, these techniques of the prior art were difficult to adapt for practical use, as they have the following problems. With the technical specifications of the Japanese Patent Examined Publication, No. S30-3139, for instance, it is difficult to form the heat insulator into a consistent shape because it is composed solely of glass fiber. In a case of fabricating a plate-form vacuum heat insulator, it raises a problem requiring an extra man-hour when using the glass fibers as a core of the vacuum heat insulator, since the glass fibers themselves are not capable of retaining its own shape.
Moreover, according to the technical specifications of the Japanese Patent Laid-Open Publication, No. S60-208226, although the fibers themselves are provided with an ability of retaining their own shape because of the sewn up fibers, there is a problem of high production cost since the conventional method is not employed to sew up the fibers while maintaining a reduced solid thermal conduction.
Furthermore, according to the technical description of the Japanese Patent Laid-Open Publication H9-138058, there is proposed a technique of fixing fibrous material with organic binder as the method of providing the fibrous material with shape retaining capability. However, though it specifies a kind of binder, it does not teach details of filling amount, material composition of the fibers used and so on. Therefore, it still has a problem leaving difficulty of fixing the fibers with binder while maintaining the insulating property necessary to use as a vacuum heat insulator.
Japanese Patent Laid-Open Publications, Nos. H07-167376 and H07-139691, for example, disclose high performance vacuum heat insulators. The Publication, No. H07-167376 teaches a technique to obtain vacuum heat insulator using a core composed of inorganic fibers having an average fiber diameter of 2 μm or smaller, and more desirably 1 μm or smaller, which are treated in an acid solution followed by a compression dehydration, so as to concentrate an eluted component of the inorganic fibers at intersecting points of the fibers, and make the component as binding material to bind the fibers. The Publication, No. H07-139691 teaches a technique to obtain vacuum heat insulator using inorganic fibers having an average fiber diameter of 2 μm or smaller, and more desirably 1 μm or smaller, in which a plurality of sheets of paper obtained from the inorganic fibers by acid paper-making process are laminated under acidic atmosphere, followed by a compression process, and the fibers are then bound at individual intersecting points with a component eluted from the inorganic fibers.
In addition, there has been proposed vacuum heat insulator having flexibility, which primarily uses fibrous material. In Japanese Patent Laid-Open Publication, No. H05-502431 for instance, there is disclosed vacuum heat insulator which contains inorganic fiber material of 1 μm or larger, but 25 μm or smaller in fiber diameter as a core, and characterized by not containing binding material. It discloses such an advantage as excellent reliability because of no degradation in heat insulation property in long term use since it contains no binding material that generate gaseous product under the vacuum condition within an enveloping member, in addition to the superiority in flexibility.
However, these techniques of the prior art have the following problems. According to the technical specifications disclosed in the Laid-Open Publications, Nos. H07-167376 and H07-139691, for instance, the vacuum heat insulators lack flexibility, and are not formed in shapes such as bent, curved, and cylindrically formed, because they use as cores the composition produced by concentrating eluted component of the inorganic fibers at intersecting points of the fibers, rendering it to act as binding material for binding the fibers, and subjecting the fibers to the compression process.
Furthermore, the technical specifications of the Laid-Open Publication, No. H05-502431 provides the vacuum heat insulator, which is adaptable to alteration in shape and superior in reliability. However, it is not satisfactory because it only has a heat insulation property of approximately three times that of the ordinary rigid urethane foam of the prior art, since it uses the fibers having diameters larger than 1 μm.
As described above, the problems common to the techniques of the prior art are the cost and performance, especially in respect of a difficulty in realizing compatibility between heat insulation property and flexibility, and thereby they are not adaptable for practical use.
The present invention is to provide vacuum heat insulator, which is practical for use while also realizing low-cost, by defining physical properties and characteristics of fiber material used for the vacuum heat insulator, as well as a kind of binder and specific filling amount of the binder when used. The invention also provides electronic apparatuses that use the vacuum heat insulator.